Implement refresh rate limiting

[nyaray]

Some ideas for how this could be done:

• A minimum amount of time between pushes
• A period for how often to check for changes

These periods should probably be configurable, with a sane default, given the amount of time it takes to redraw. Another option could be to have the driver check if it's been pushing pixels with timeouts set as the policy for too long.

Maybe an additional :push value, {:update_after, time}, would be enough to ensure that we draw, but don't flood the display with updates?